1. Field of the Invention
The invention relates to an apparatus and a method for detecting an abnormality of inter-cylinder air-fuel ratio dispersion in a multi-cylinder internal combustion engine, and more particularly, to an apparatus and a method for detecting that air-fuel ratios relatively greatly disperse among cylinders in a multi-cylinder internal combustion engine.
2. Description of the Related Art
In general, in an internal combustion engine equipped with an exhaust gas purification system utilizing a catalyst, the control of the mixing ratio between air and fuel in the mixture burned in the internal combustion engine, namely, the air-fuel ratio is indispensable, with a view to purifying exhaust gas of noxious components contained therein by a catalyst with high efficiency. In order to perform this air-fuel ratio control, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and feedback control is performed such that the air-fuel ratio detected by the air-fuel ratio sensor coincides with a predetermined target air-fuel ratio.
On the other hand, in a multi-cylinder internal combustion engine, the same control amount is usually used for all cylinders to perform air-fuel ratio control. Therefore, the actual air-fuel ratios may disperse among the cylinders even when air-fuel ratio control is performed. When the degree of dispersion is low in this case, the dispersion can be absorbed through air-fuel ratio feedback control, and exhaust gas can be purified of noxious components contained therein by the catalyst as well. Thus, the exhaust emission properties are not influenced, and no particular problem arises. However, when the air-fuel ratios greatly disperse among the cylinders due to, for example, a malfunction of a fuel injection system in one or each of some of the cylinders, the exhaust emission properties deteriorate to cause a problem. Such great air-fuel ratio dispersion as deteriorates the exhaust emission properties is desired to be detected as an abnormality. Especially in the case of an internal combustion engine for an automobile, in order to prevent in advance a vehicle with deteriorated exhaust emission properties from running, there is a demand to detect an abnormality of inter-cylinder air-fuel ratio dispersion in an on-vehicle (onboard) state. Recently there have also been moves to legislate against this abnormality.
Japanese Patent Application Publication No. 2000-220489 (JP-A-2000-220489) discloses a control apparatus for a multi-cylinder engine having a single air-fuel ratio sensor disposed in an exhaust pipe aggregate portion of the engine which calculates individual air-fuel ratios in respective cylinders and controls air-fuel ratios in the cylinders respectively. According to this control apparatus, an air-fuel ratio is calculated on the basis of an output signal of the air-fuel ratio sensor and then analyzed into frequency components within a predetermined range. An air-fuel ratio in each of the cylinders is estimated on the basis of these analyzed frequency components.
When air-fuel ratios in the respective cylinders can be estimated as in the case of the apparatus described in Japanese Patent Application Publication No. 2000-220489 (JP-A-2000-220489), an abnormality of air-fuel ratio dispersion may be detected by comparing these air-fuel ratios with one another. However, in the apparatus described in Japanese Patent Application Publication No. 2000-220489 (JP-A-2000-220489), fluctuations in the air-fuel ratio that are synchronized with the rotation of the engine need to be detected by the air-fuel ratio sensor on a short cycle. Thus, the air-fuel ratio sensor is required to exhibit very high responsiveness. Even if there is such a sensor, it may fail to function when it degrades and decreases in responsiveness. Further, an ECU with high-speed data sample or high processing capacity is required. It is difficult for a high-responsiveness sensor to detect only fluctuations in the air-fuel ratio separately from noise, and there is a lack of robustness. There is also a restriction on the condition for operating the engine as well. For example, the engine is operated only in a stationary state so as to minimize disturbances. To detect fluctuations in the air-fuel ratio that are synchronized with the rotation of the engine, it is preferable to install the sensor as close as possible to a combustion chamber. In this case, however, there is an apprehension that a sensor element may be damaged by moisture in exhaust gas. The shape of an exhaust manifold and the installation position of the sensor need to be selected such that a good amount of gas flows onto the air-fuel ratio sensor. Even when air-fuel ratios in the respective cylinders are estimated to attempt to detect an abnormality of air-fuel ratio dispersion, there are still many problems to be solved. It is therefore difficult to say that this control apparatus is practical.